As depicted in FIG. 1, a typical shoulder or glenohumeral joint is formed in a human body where the humerus 10 movably contacts the scapula 12. The scapula 12 includes a glenoid fossa 14 that forms a socket against which the head of the humerus 10 articulates. At this socket, the scapula 12 includes cartilage 16 that facilitates such articulation. Beneath the cartilage is subchondral bone 18 that forms a wall of a glenoid vault 20 that defines a cavity which contains cancellous bone 22. The subchondral bone 18 that forms the glenoid vault 20 defines a glenoid rim 24 at a periphery of the glenoid vault 20 that is attached to the cartilage 16. During the lifetime of a patient, the glenoid fossa 14 may become worn, especially at its posterior and/or superior portions thereby causing severe shoulder pain and limiting the range of motion of the patient's shoulder joint. To alleviate such pain and increase the patient's range of motion, a shoulder arthroplasty may be performed. Arthroplasty is the surgical replacement of one or more bone structures of a joint with one or more prostheses.
Shoulder arthroplasty often involves replacement of the glenoid fossa of the scapula with a prosthetic glenoid component. The conventional glenoid component typically provides a generally laterally or outwardly facing generally concave bearing surface against which a prosthetic humeral head (or, alternatively, the spared natural humeral head in the case of a glenoid hemi-arthroplasty) may bear during operation of the joint. The conventional glenoid component typically also includes a generally medially or inwardly projecting stem for fixing the glenoid component in a cavity constructed by suitably resecting the glenoid fossa 14 and suitably resecting cancellous bone 22 from the glenoid vault 20.
The goal of shoulder arthroplasty is to restore normal kinematics to the shoulder. Accordingly, known systems attempt to replicate the normal kinematics by carefully controlling the geometry of the articulating surfaces in the joint as well as the positioning of the prostheses in the bones in which the prostheses are implanted. Thus, the articulating surface of a humeral component is typically spherical and positioning of the humeral component is accomplished by using the anatomical neck of the humerus as the reference plane for reconstruction of the humeral head.
In known systems, the glenoid component is positioned in the geometric center of the glenoid fossa. The geometric center is established by generating a line from the most superior point of the glenoid rim to the most inferior point of the glenoid rim (“Saller's line”). A second line is generated between the most posterior point of the glenoid rim and the most anterior point of the glenoid rim. The intersection of the two generated lines is considered to be the geometric center of the area circumscribed by the glenoid rim. By way of example, FIG. 2 depicts a sagittal view of the scapula 12. In FIG. 2, Saller's line 30 extends between the most superior point 32 of the glenoid rim 24 and the most inferior point 34 of the glenoid rim 24. A second line 36 extends from the most posterior point 38 of the glenoid rim 24 and the most anterior point 40 of the glenoid rim. The geometric center 42 of the glenoid fossa 14 is located at the intersection of the line 36 and Saller's line 30. As used herein, the terms anterior, posterior, superior, and inferior, unless otherwise specifically described, are used with respect to the orientation of the scapula 12 as depicted in FIG. 2.
Depending upon the particular morphology of an individual, preparing a glenoid to receive an implant once the desired location for the implant is determined can be problematic. One particularly difficult morphology is referred to as “glenoid retroversion.” In glenoid retroversion, the glenoid fossa has experienced uneven deterioration. Since known glenoid components are configured to be implanted upon a level base of bone, significant surface preparation is required before the glenoid component can be implanted. In some approaches, a bone graft is used to even out the glenoid fossa so that a desired orientation of a glenoid implant can be achieved. Using a bone grafting procedure, however, is typically a two-stage procedure which complicates the implant procedure.
In another approach, the area of the glenoid fossa in which the implant is to be implanted is simply planarized to the level of the deepest defect and a thicker glenoid implant is used to achieve the desired position for the articulating surface of the glenoid component. While effective, this approach necessitates removal of a large amount of bone.
There remains a need for a glenoid component that allows for establishing normal kinematics. There is a further need for a technique, instrumentation, and implant that facilitates positioning of such a component. A glenoid component that can be positioned in a manner that reduces the amount of bone that is required to be removed without overly complicating the implant procedure is also needed.